1. Field of the Invention
The present invention relates to a recording head for electrostatic recording which forms an electrostatic latent image by modulating ion currents and selectively introducing ions into a dielectric substance and to a manufacturing method thereof.
2. Discussion of the Related Art
An electrostatic recording apparatus uses a recording head capable of modulating ion currents corresponding to each picture element to form an electrostatic latent image of a desired pattern on a dielectric substance. For example, U.S. Pat. No. 4,408,214 discloses the construction of a recording head in which a matrix is constituted by an intersecting plurality of drive electrodes and plurality of control electrodes with an interposed insulating layer to form an electrostatic latent image corresponding to picture elements.
Japanese Patent Application Unexamined Publication No. Hei. 3-109585 (1991) discloses another example: an electrostatic recording apparatus using a recording head in which an insulating layer between electrodes is made of a ceramic material.
In the above recording apparatus disclosed by the Japanese Patent Application Unexamined Publication, as shown in FIG. 16, a recording head 101 disposed facing the surface of a dielectric drum 103 forms an electrostatic latent image on the dielectric drum 103 in accordance with an image signal. The electrostatic latent image is developed by a developing device 102, and then transferred and fused by a pressure roller 107 to a recording medium 106 such as recording paper, thus recording the image. Developer remaining on the dielectric drum 103 after transfer is removed by a cleaning device 104 and residual toner on the dielectric drum 103 is removed by a discharging device 105.
The recording head 101 has a plurality of ion emission ports each of them corresponding to one picture element. FIG. 17 is a cross-sectional view showing the ion emission port for one picture element. FIG. 18 is a plan view showing the whole recording head 101 except a screen electrode.
In the recording head 101, plural lower electrodes 2-1, 2-2, and so on (hereafter, generically referred to as the lower electrodes 2) are arranged parallel to each other on a substrate 1, and forked upper electrodes 4-1, 4-2, and so on (generically referred to as the upper electrodes 4) are arranged on and crossing the lower electrodes 2 with a lower insulating layer 3 between, thus the lower electrodes 2 and upper electrodes 4 constitute a matrix. In the example shown in FIG. 18, the number of the lower electrodes arranged on the substrate is n.
The upper electrodes 4 have apertures 4a at the point of intersection with the lower electrodes 2, and a screen electrode 22 is mounted on an insulating layer 10 on the upper electrodes 4. The insulating layer 10 and screen electrode 22 have apertures 10a and 22a, respectively, at the points corresponding to the apertures 4a in the upper electrodes 4.
As shown in FIG. 17, the above-described recording head 101 selectively applies a high voltage of high frequency V.sub.B between the lower electrodes 2 and screen electrode 22, and applies an ion control voltage V.sub.C to the upper electrodes 4, and a DC voltage V.sub.A to the screen electrode 22, respectively.
Consequently, creeping corona discharge is generated between the lower electrodes 2 and upper electrodes 4 corresponding to a matrix to which the high voltage of high frequency is applied, and ions generated by the creeping corona discharge are accelerated or absorbed by the electric field between the upper electrodes 4 and the screen electrode 22 to control ion emission to obtain the electrostatic latent image.
The recording head 101 described above is made, for example, by the following print-lamination method.
First of all, the lower electrodes 2 which function as drive electrodes are formed by screen printing or the like on a substrate 1 of a material called green sheet which can be hardened by firing, and the lower insulating layer 3 made of the green sheet is formed by screen printing or the like on the substrate on which the lower electrodes 2 are already formed. The upper electrodes 4 which function as control electrodes are formed on the lower insulating layer 3 by screen printing or the like, and the upper insulating layer 5 is formed thereon by screen printing or the like. After that the green sheet, upper and lower electrodes and insulating layers are fired. The green sheet material will be described in detail later.
However, the above-described method for making a recording head has a problem that because the green sheet material and electrodes in layers contract when fired, the dimensions of the recording head after firing are reduced. To obtain a recording head of the desired dimensions after firing, the electrodes and so forth are printed taking the contraction rate into account, but it is still difficult to obtain the head of desired dimensions after firing because of the wide variation of the contraction rate approximately ranging from 12 to 20%.
A conventional method for manufacturing a recording head completes the recording head by fitting the screen electrode 22 after firing. When fitting the screen electrode, a large offset between the positions of the aperture 22a of the screen electrode 22 and the ion emission port consisting of apertures 4a and 10a causes a reduction of the output and results in deterioration of the recorded image density, unevenness or blurring of the image. It is preferable that the offset is not more than one half of the recording resolution, for example, 50 .mu.m or less for 240 dpi, and 40 .mu.m or less for 300 dpi. However, because the contraction rate of the recording head caused by firing is considerable, approximately ranging from 12 to 20%, it is difficult to restrict the offset to one half or less of the recording resolution.